1. Field of the Invention
The present invention relates to a light-emitting apparatus.
2. Description of the Related Art
An organic electroluminescent (EL) display, being a kind of light-emitting apparatus, is a new type of flat panel display and formed from an array of organic light-emitting diode (OLED) devices. In general, an OLED device is comprised of an electrode serving as the anode, an electrode serving as the cathode and several thin organic layers sandwiched between these two electrodes. The organic layers include at least one emission layer containing more than one fluorescent or phosphorescent organic compounds to emit light of each emission color. When applying a voltage to the OLED device, holes and electrons are injected from the anode and the cathode, respectively, and form excitons in the emission layer. Then these excitons recombine and release their energy as emission of light. According to a research conducted by Eastman Kodak Company in 1987, which is reported in Appl. Phys. Lett. 51, 913 (1987), a device with a two-layered structure is proposed, and light emission of about 1,000 cd/m2 at an applied voltage of about 10 V is reported. ITO is used for an anode, an Ag—Mg alloy is used for a cathode, an aluminum quinolinol complex is used as an electron-transporting and emission layer, and a triphenyl amine derivative is used for a hole-transporting layer.
As one of problems involved in the development of an organic light-emitting apparatus, there is a need for reduction in power consumption. The OLED device generally has a structure in which functional layers including an emission layer are stacked one-dimensionally. In this case, the refractive index (about 1.7 or more) of the emission layer is larger than that of the air. Therefore, most of light emitted from an inside of the emission layer becomes guided-wave light and is reflected totally. Therefore, most of light emitted from an inside of the emission layer is trapped in guided-wave light in the device by total reflection at an interface of the stacked film at which the refractive index changes from high to low. The percentage (light extraction efficiency) of light that can be extracted and utilized of the light generated inside the emission layer is generally merely about 20%.
Thus, in order to reduce the power consumption of the organic light-emitting apparatus, it is important to enhance the light extraction efficiency to improve the emission efficiency of the OLED device. It has been proposed in U.S. Pat. No. 5,779,924 and Japanese Patent Application Laid-Open No. 2004-349111 that in order to prevent total reflection and suppress the light confinement inside the device, periodic structures (a subwavelength periodic structure, a diffraction grating, etc.) is placed in upper and lower portions (on a light extraction side and on an opposite side thereof) of the functional layer.
As another method, in Japanese Patent Application Laid-Open No. H11-214163, there has been proposed that in order to reflect, in a light extraction direction, the light which is confined in a device as a guided-wave light and leaks out through a side surface of the device, an inclined metallic reflective surface is disposed at a device side surface to improve the light extraction efficiency.
According to the technologies disclosed in U.S. Pat. No. 5,779,924 and Japanese Patent Application Laid-Open No. 2004-349111 above, in the case of disposing a periodic structure at an upper portion of a functional layer, it is necessary to make the periodic structure after formation of the functional layer, so that there is a problem that the functional layer may be damaged during the making process of the periodic structure.
Further, in the case of disposing a periodic structure at a lower portion of a functional layer, because the interface between the functional layer and an electrode, or the like is not flat, there is a problem that non-uniformity and reduction in adhesiveness of a film and the like occur during formation of the film by vacuum vapor deposition, which may degrade the device characteristics and the device durability.
According to the technology disclosed in Patent Application Laid-Open No. H11-214163 above, because the distance between electrodes varies between when measured at a center portion and when measured at a peripheral portion of the device due to the inclination of the metal electrode. Therefore, there is a problem of degradation of the device durability caused by local light emission and increase of the number of non-light-emitting devices due to short-circuiting between the electrodes.